1. Field of the Invention
The invention relates to a multiple quantum well distributed feedback (DFB) semiconductor laser device suitable for a coherent optical transmission system and a method for fabricating the same.
2. Description of the Related Art
The coherent optical communication system is more attractive than a direct detecting system as showing a high receiving sensitivity which is suitable for a long distance optical transmission. The most realizable variety of the coherent optical transmission system seems to be a frequency shift keying (FSK) coherent optical transmission as it has a simple transmitter structure system in which a direct modulation is conducted for the semiconductor frequency. The laser device as an optical transmitter used for the optical coherent transmission is required to show a single mode oscillation or a stability of the single wavelength oscillation at high output. The laser device for the optical coherent transmission is further required to show a high efficiency of a frequency modulation (FM) and a uniform or flat frequency modulation response in a wide modulation frequency range as well as a narrow spectral linewidth. The distributed feedback laser is useful as the suitable laser device for the optical coherent transmission. The distributed feedback semiconductor laser device has a waveguide and a corrugation-shaped grating formed over or under the waveguide to accomplish feedback of optical wave. An oscillation wavelength of a laser beam is defined by almost only a pitch or a period of the corrugation shaped grating thereby the stable single mode oscillation is obtained. Actually, however, two resonant vertical modes exist at opposite sides of the wavelength defined by the corrugation-shaped grating. There is a problem with simultaneous appearances of oscillations in the two resonant vertical modes. There is a further problem with a variation of the wavelength in a high optical output.
To solve the above problems, it is necessary to cause a resonance at the center of the Bragg wavelengths. For that purpose, it has been known to provide a .lambda./4 shift at the center of the corrugation shaped grating. The .lambda./4 shift causes a phase shift of .pi.. The distributed feedback laser with the .lambda./4 shift has properties of a narrow spectral linewidth of less than 1 MHz, an optical output of more than 30 mW and a frequency modulation efficiency of approximately 200 MHz/mA. One of such the distributed feedback laser with the .lambda./4 shift is disclosed in 1991 IEEE Photonics Technology Letters vol. 3, No.4, pp. 305-307.
Such distributed feedback laser with a .lambda./4 shift, however, has a disadvantage because it is difficult to fabricate the phase shift grating, although the phase shift grating may be fabricated by use of either multilayer photoresist mask or electron beam and the like. The distributed feedback laser with the .lambda./4 shift has further problems as described below. A distribution of the optical intensity tends to be concentrated around the phase shift grating. As the optical output is increased, a carrier density around the phase shift grating is reduced as compared to the peripheral portion of the phase shift grating thereby a stable single mode oscillation is no longer obtainable. This further causes a rapid reduction of the frequency modulation efficiency in a high modulation frequency range more than approximately 1 GHz. This further more causes a broadening of the spectral linewidth due to a hole burning effect along a cavity length direction of the distributed feedback laser device. To combat the above mentioned problem, it is necessary to suppress the hole burning effect so as to make the distribution of the optical intensity be uniform or flat along the resonance direction or the cavity length direction. It is thus important that the distributed feedback laser with the phase shift grating possesses the uniform or flat distribution of the electronic field of the light to obtain the stable single mode oscillation at the high optical output as well as the flat or uniform frequency modulation properties.
To achieve the above purpose, a corrugation-pitch-modulated distributed feedback laser was proposed and disclosed at the 1991 Conference on Laser and Electro-Optics 654/CPD 40-1. The distributed feedback laser device has a corrugation pitch modulated grating having a larger pitch or a larger period thereof in the center area of about 100 micrometers and a small pitch or a small period outside the center area. The above laser device is able to show a narrow spectral linewidth less than about 100 KHz and an optical output of about 30 mW. The above laser device, however, has a disadvantage in a difficulty in a fabrication of such the corrugation pitch modulated grating. To suppress the hole burning effect, the corrugation pitch modulated grating is required to have a slight difference of angstrom order in the pitches or in the periods thereof between in the center area and the other areas outside the center area. The formation of the corrugation pitch modulated grating requires specific means such as electron beam lithography or ruling engine which provides a difficulty in the fabrication process thereof. The distributed feedback laser with the corrugation pitch modulated grating is, necessarily, engaged with a problem of the difficulty in the fabrication of the corrugation-pitch-modulated grating.
Other type of the distributed feedback laser device with a .lambda./4 shift was proposed and disclosed in the 1990 IEEE Photonics Technology Letters, Vol. 2, No. 3, pp. 165-166. The proposed distributed feedback laser device has an n-side electrode formed under a substrate and a p-side electrode formed on the top of the laser device. To control the distribution of the light intensities along the cavity length direction or the resonance direction, the p-side electrode is divided into three sections which form a line along the cavity length direction or a resonance direction. The distributed feedback laser device having the p-side electrode divided into the three sections shows properties of a spectral linewidth less than about 1 MHz, an optical output of about 20 mW, a frequency modulation range less than 15 GHz and a frequency modulation efficiency of about 600 MHz/mA.
Alternatively, a corrugation-pitch-modulated ditributed feedback laser device having a p-side electrode which is divided into three sections was proposed and disclosed in the Fourth Optoelectronics Conference Technical Digest, July 1992, pp. 58-59. The corrugation-pitch-modulated ditributed feedback laser device has an n-side electrode formed under a substrate and the p-side electrode formed on the top of the laser device. To control the distribution of the light intensities along the cavity length direction or the resonance direction, the p-side electrode is divided into three sections which form a line along the cavity length direction or a resonance direction. The corrugation pitch modulated distributed feedback laser device having the p-side electrode divided into the three sections shows properties of a spectral linewidth less than 100 kHz and a variable wavelength range of 1 nanometer.
The above two types of the distributed feedback laser device with the electrode divided into the three sections, however, have a disadvantage in that it is difficult to control the voltages to be applied to the three sections as the electrode. Any small variation of the biases or voltages applied to the three sections as the p-side electrode unfortunately causes a great variation of oscillating performance of the laser device. To obtain a desirable oscillating performance, it is essential to make a very exact control of the voltages applied to the three sections as the p-side electrode of the laser device. Particularly when the distributed feedback laser device with the multiple sectioned electrode is applied to a high speed system greater than 10 Gb/s, it is necessary to conduct a frequency deviation in the same level as the frequency modulation under a current amplitude sufficient for driving high speed integrated circuits. This requires a much higher frequency modulation efficiency and uniform response to the frequency modulation in a wader range of the modulation wavelengths. Actually, it is difficult for the conventional distributed feedback laser with the multiple sectioned electrode to satisfy the above requirements.
It has been known as one of methods for improving the efficiency of the frequency modulation to utilize a gain lever effect. The laser device utilizing the gain lever effect which shows a high frequency modulation efficiency of about 20 GHz/mA was proposed and disclosed in 1990, in the Applied Physics Letters Vol. 57 No. 20, pp. 2068-2070. The gain lever effect shows a large change in the carrier density in a gain section of the laser device by a small change in injection current in a control section based on a highly sublinear nature of the gain versus carrier density characteristics of a single quantum well laser device. The gain lever effect may be useful to enhance the frequency modulation efficiency of the laser device without a corresponding increase in the frequency modulation noise and broadening of the spectral linewidth and the like.
The gain lever effect may also appear in a semiconductor laser device with an electrode divided into two or more sections and an active layer whose composition is uniform. The frequency modulation response of tunable two-segment distributed feedback laser device was reported in the 1989 Applied Physics Letters Vol. 55 No. 18, pp. 1826-1828. The electrode are divided into two segments to be applied with injection currents which are asymmetric to each other. The first and second segments as the electrode are applied with a small injection current and a large injection current respectively. In a first area including the first segment applied to the small injection current, a differential gain coefficient is large. In contrast, in a second area including the second segment applied to the large injection current, the differential gain coefficient is small. The above phenomenons are due to that the differential gain coefficient depends upon the carrier density. Namely, a slight modulation of the small injection current to be applied to the first segment causes a large variation of the carrier density in the second area including the second segment applied with the large injection current so as to obtain a high efficiency of the frequency modulation. The above effect may be considered as one kind of the gain lever effects.
The above distributed feedback laser device utilizing the above mentioned gain lever effect with the two segment electrode has, however, disadvantages in a long carrier lifetime in the small injection current area of the laser device as well as a reduction of the light intensity. Those makes the frequency modulation range be narrow.